1. Field of the Invention
The present invention relates to an optical communication, and more particularly, to a semiconductor quantum dot optical amplifier that has not a polarization dependency and a signal leakage between channels but has a wide gain band by forming a quantum dot active layer between clad layers, an optical amplifier module which has a feature suitable for a WDM optical communication system and has a wide gain band including a wavelength band which could not be amplified by a conventional optical amplifier, and an optical transmission system which is formed by connecting several optical amplifiers using the optical amplifier module and that a gain flatness is automatically performed and a gain is automatically fixed in spite of a variation of channel.
2. Description of Related Art
As a communication technology has been developed, an implementation of a WDM optical communication system has become possible and, thus a communication capacity has also been significantly increased. However, in spite of such a development of the communication technology, 70 nm band which is a currently available band of an optical fiber amplifier cannot keep pace with a demand of rapidly increasing communication capacity. Hence, there is a need for an optical amplifier which can use an entire band range of 1250 nm to 1650 nm which is a low absorption band.
A conventional rare earth doped optical fiber amplifier has a feature that a gain band is determined by a corresponding rare earth element, but now other rare earth doped optical fiber amplifiers except the erbium doped optical fiber amplifier are inefficient.
On the other hands, a semiconductor optical amplifier has an advantage in that a gain band can be selected by adjusting an energy gap. Therefore, if such a semiconductor optical amplifier is used as an optical amplifier for an optical communication, it is expected to solve a problem resulting from a rapidly increasing communication capacity because a desired wavelength band is obtained.
FIG. 1 is a cross-sectional view illustrating a conventional semiconductor optical amplifier having an active layer made of a quantum well, and FIG. 2 is a measurement result view to show a polarization dependency according to a gain in the conventional semiconductor optical amplifier.
As shown in FIG. 1, the conventional semiconductor optical amplifier includes a semiconductor substrate 10. A first conductive type clad layer 12, an active layer 14, is and a second conductive type clad layer 16 are stacked on the semiconductor substrate 10 in this order. The active layer 14 is made of a quantum well. A light signal represented by a left-side small arrow incident to one side of the conventional semiconductor amplifier is amplified while passing through a quantum well active layer 14 and then go out of the other side thereof.
However, the conventional semiconductor optical amplifier having the active layer 14 of the quantum well has a relatively small homogeneous broadening of a corresponding energy level. Therefore, in the case of the optical communication system which several adjacent wavelengths are amplified such as a WDM system, a gain interference between adjacent channels becomes severe and thus it is impossible to be put to practical use.
FIG. 2 is a resultant view of a gain spectrum according a polarization of an input light signal of the convention semiconductor optical amplifier.
As shown in FIG. 2, since the conventional semiconductor optical amplifier has a gain greatly depending on a polarization of an input light [M. Asada, A, Kameyama, and Y. Suematsu, “Gain and intervalence band absorption in quantum-well lasers”, IEEE J. Quantum Electronics, QE-2, 745–753 (1984)], an optical transmission system that a polarization varies irregularly according to a time has a problem in that a size of a gain varies irregularly and thus is almost impossible to be used a practical system. Also, a system that several wavelengths are amplified at a time such as a WDM optical communication system is impossible to be used as an optical amplifier for a communication because a gain interference between channels is severe due to a large homogeneous broadening.
In recent, an optical communication of a WDM method that one optical fiber has a number of channels having different wavelengths has been introduced, and this makes a communication capacity increase.
FIG. 3 is a configuration view of a conventional long-distance WDM optical communication, and FIG. 4 is a configuration view of an optical transmission system of a conventional WDM network.
As shown in FIGS. 3 and 4, in such a WDM system, several wavelengths are amplified at a time and several amplifiers 1101, 1102, . . . , 110N are used. However, in this case, when gains to wavelengths are not identical, there occurs a problem in that a difference of a light intensity between channels becomes large at a last receiving end. That is, since there occurs a phenomenon that a gain at each wavelength is multiplied when it passes through several amplifiers 1101, 1102, . . . , 110N, even though at the beginning gains are approximate, a gain difference increase as much as the number of amplifiers through which it passes.
As shown in FIG. 4, in the case of the conventional WDM network, part of channels are added or dropped, and thus a channel number is varied, leading to a variation of input signal intensity. In the case of the conventional optical fiber amplifier, since a gain spectrum depends on an intensity of the input signal, an optical amplifier having a flatted gain has a problem in receiving signal since a signal intensity greatly depends on a channel and gains in remaining channels are also greatly varied.
FIG. 5 is a view illustrating a gain spectrum measurement result in an optical transmission system connected to a convention erbium doped optical fiber amplifier. In FIG. 5, a large signal of 1550 nm denotes an amplified input light, and a line laid over a wide region denotes a gain flatness. A curve A represents a gain result when one amplifier is used for amplification, and a curve B represents a gain result when 30 amplifiers are used for amplification. FIG. 5 shows a problem occurring when a signal light passes through several amplifiers 1101, 1102, . . . , 110N. That is, a gain difference in 1532 nm and 1557 nm is within 1 dB when it passes through one amplifier, while a gain difference is 24 dB when it passes through 30 amplifiers. That is, a signal light
Such a feature occurs because the erbium doped optical fiber amplifier is large in tendency of a homogeneous broadening. FIG. 6 is a configuration view of a conventional gain-flattening optical fiber amplifier, and shows the optical amplifier which is designed to have a flat gain such that a gain-flattening filter 145 is inserted to permit an appropriate loss in order to overcome the problem.
In this case, however, it is very difficult to precisely adjust a gain in one amplifier. In addition, even though a gain difference is within 1 dB in one amplifier which have performed the gain adjustment, when it passes through several amplifiers, for example, 100 amplifiers, a gain difference of tens of dBs occurs. Also, when an intensity of an input signal or a channel number is varied, the optical amplifier having the gain-flattening filter 145 has a problem in that a gain flatness of a designed optical amplifier becomes bad.
The optical communication of the WDM method can put a large number of channels having different wavelengths in one optical fiber and thus can rapidly increase a communication capacity. Therefore, it can sent more signal amounts as a gain band is wider. It is expected that a current available band of an optical amplifier, i.e., 70 nm, is soon difficult to keep pace to a rapidly increasing communication capacity. Therefore, there is a need for an optical amplifier which can use all region of 1250 nm to 1650 nm which are a low absorption band of an optical fiber.
FIG. 7 is a view illustrating a gain spectrum result of a conventional Raman optical fiber amplifier.
In order to solve the problems, the conventional Raman optical fiber amplifier use a non-linear phenomenon of an optical fiber to broaden an amplification band and is used to increase a communication capacity. The conventional Raman optical fiber amplifier requires an excitation light of about 1 W in a single mode optical fiber to obtain a sufficient gain, but such a light source is large in size and high in power consumption and thus is not of practical use. Also, an amplification band obtained from one light source is about 20 nm as shown in FIG. 7. Thus, a large number of high-power elements are additional required to obtain an amplification gain of a broad region and thus there are many difficult in using the system.
Besides, as a transmission capacity is increased, there is an expectation that an optical amplifier system of such a broad gain band is required in future. The optical amplifier system of such a broad gain band is necessarily required in a repeater for a long-distance transmission, a post-amplifier of a transmitting end, a pre-amplifier of a receiving end, and a metro WDM system.